1. Field of the Invention
The present invention relates to a laminated coil and a method for producing the laminated coil. More particularly, the present invention relates to the shape of via holes in a laminated coil and a method for forming such via holes.
2. Description of the Related Art
As an example of a laminated coil, a chip inductor disclosed in Japanese Unexamined Patent Application Publication No. 2002-252117 is widely known, and the construction of the chip inductor is shown in FIG. 9, and FIG. 10 is an exploded perspective view thereof. As shown in FIGS. 9 and 10, a related vertical lamination horizontal winding type chip inductor 11 has a structure in which a coil 13 wound in the direction Y, which is perpendicular to the lamination direction X of a laminated body 12, is disposed inside the laminated body 12. The coil 13 is constructed such that conductor patterns (belt-shaped conductors) 14 formed on laminated surfaces at fixed locations on the upper portion and the lower portion of the laminated body 12 are electrically connected through many via holes 15. Many of the via holes 15 are formed to extend in the lamination direction X.
That is, as shown in FIG. 10, these via holes 15 are formed such that through holes 17 are formed at fixed locations on each of ceramic green sheets 16 by laser radiation, etc., and these through-holes 17 are filled with a conductor such as conductor paste, etc. Then, as shown in FIGS. 11 and 12, each through-hole 17 has a substantially round flat shape and its inner surface has the same angle of inclination (taper angle) along the lamination direction X. Moreover, the ceramic green sheets 16 constitute ceramic layers in the laminated body 12.
FIG. 11 is a top view of the through-holes 17 and FIG. 12 shows the section of the through-holes 17 taken along line A—A in FIG. 11. That is, each through-hole 17 is constructed such that the diameter of the upper opening 17b is larger than the diameter of the lower opening 17a. Furthermore, the conductor patterns formed at the end portions on the upper surface of the laminated body 12 are lead to the end surfaces and connected to external electrodes 18 formed so as to cover the end surfaces of the laminated body 12, respectively.
On the other hand, when the laminated body 12 is produced, many of the ceramic green sheets in which only via holes are formed are disposed in the middle of the lamination direction X. Then, plural ceramic green sheets 16 in which conductor patterns 14 and via holes 15 are formed are disposed above and below the above-described ceramic green sheets 16. Furthermore, plural ceramic green sheets 16 in which no conductor patterns 14 or via holes 15 are formed are disposed above and below the above-described ceramic green sheets 16. Then, the ceramic green sheets 16 are attached by pressure in the lamination direction X and fired to obtain a laminated body 12. When the external electrodes 18 are formed on the end surfaces of the laminated body 12, a chip inductor 11 shown in FIG. 9 is completed.
In the chip inductor 11, since many via holes 15 are formed, the ratio of the resistance Rdc of the portion in which the via holes 15 are formed to the DC resistance value Rdc of the whole chip inductor 11 increases. It is not avoidable that the resistance Rdc of the whole element is affected. Thus, in order to prevent such a drawback, it is possible to consider that the flat shape of the via holes 15 is increased and, as a result, the inner volume of the via holes 15 is increased.
However, when the flat shape of the via holes 15 is simply increased, since the flat shape of the via holes 15 is substantially round, the spacing between adjacent via holes 15 in the axial direction of the coil 13 is narrowed. Furthermore, when the flat shape of the via holes 15 is made larger and the spacing between via holes 15 is appropriately kept, the number of turns of the coil 13 is reduced. As a result, a large impedance cannot be obtained.